This invention relates to a projection exposure apparatus used when manufacturing a semiconductor chip, a liquid crystal panel, a CCD, a thin-film magnetic head or a micromachine, etc., by means of photolithography. More particularly, the invention relates to a projection exposure apparatus having means for adjusting deviation between the substrate surface of a mask or the substrate surface of a semiconductor wafer or the like, which is for manufacturing a device such as a semiconductor device or liquid crystal device, and the imaging plane of a projection optical system. The invention further relates to a method of manufacturing a device.
When a semiconductor device or liquid crystal device is manufactured by photolithography, use is made of a semiconductor exposure apparatus to transfer a pattern, which is drawn on a reticle serving as a original plate, onto a wafer coated with a photosensitive material.
A so-called step-and-repeat demagnifying-projection-type semiconductor exposure apparatus is employed at many production facilities as the semiconductor exposure apparatus according to the prior art. Such an apparatus moves each of a plurality of exposure areas (shot areas) of a wafer into the exposure field of the projection optics in successive fashion and exposes each of the shot areas to a reticle circuit pattern in one batch. However, enlarging the size of semiconductor chips has become the trend in recent years and, as a result, there is a growing demand for a larger exposure surface area in a semiconductor exposure apparatus in order that a pattern of a larger area on a reticle may be transferred to a wafer. At the same time, there is a need to improve the resolving power in order to deal with finer patters on semiconductor devices. A problem with the prior art, however, is that it is difficult technically to design and manufacture a demagnifying-projection-type semiconductor exposure apparatus that satisfies both the requirements of improved resolving power and larger exposure area.
In order to solve this problem, a scanning-type projection exposure apparatus has been developed. This apparatus successively exposes a wafer to a pattern image, which has been drawn on a reticle, by scanning the reticle with respect to a slit-shaped illumination area and scanning the wafer at the same time as the reticle with respect to an exposure area serving also as the illumination area. In the scanning-type projection exposure apparatus, a reticle stage holding the reticle and a wafer stage holding the wafer, which is a photosensitive substrate, are synchronously scanned relative to the projection optics in a mutually opposing direction and at a velocity ratio that conforms to the projection magnification, thereby exposing the wafer to light.
In order to improve the throughput in both types of semiconductor exposure apparatus, the reticle stage and wafer stage must be driven at a high acceleration and high speed. Accordingly, it has not been possible to avoid vibration of structural members including a lens barrel (referred to as the projection optical system or projection optics below), which accommodates a group of projection lenses. Unfortunately, such vibration lengthens the time needed for stabilization of positioning or scanning and degrades exposure performance.
First, it is noteworthy that vibration of structural members brought about by a driving reaction force disturbs positioning stabilization of the reticle or wafer. The reason for this is that a laser interferometer serving as position measurement means for measuring the position of the reticle or wafer stage is mounted on a structural member. More specifically, since the position measurement means vibrates owing to vibration of the structural member, each stage also vibrates, as a result of which it takes longer for positioning to stabilize. Accordingly, positioning stabilization is facilitated by feeding back, to the inputs of the respective stage drivers, outputs from acceleration sensors provided on structural members in close proximity to the stages. This technique is already known and is disclosed in detail in the specification of Japanese Patent Application Laid-Open (KOKAI) No. 10-12513 entitled xe2x80x9cScanning-type Projection Exposure Apparatusxe2x80x9d. This control technique ascertains the vibration of a surface plate caused by stage vibration and moves the stage while following up this vibration, thereby eliminating positioning error.
Described next will be degradation of exposure performance caused by vibration due to the driving reaction force.
First, vibration due to the driving reaction force causes vibration not only of structural members such as the surface plate directly supporting the stage itself but also of the projection optical system, which is the heart of the projection apparatus. The mode of vibration of the projection optical system naturally is different from that of a structural member such as the surface plate directly supporting the stage. The projection optical system, which is a generally columnar structural member, usually is disposed in a vertical attitude and is mechanically connected to a structural member of the main body at a connecting portion referred to as a flange. Accordingly, a lower order mode of vibration that has a great effect upon exposure performance is one that would cause the columnar projection optical system to wobble. For example, in a case where a semiconductor exposure apparatus is of the scanning type, it is important that the wafer be exposed to a quiescent reticle circuit pattern by synchronously scanning the reticle and wafer at a predetermined velocity ratio. If the projection optical system is vibrating at this time, the circuit pattern also will vibrate on the wafer and, hence, projection precision will decline.
The effects of vibration of the projection optics, which is caused by stage vibration, on exposure precision will now be described with reference to the drawings.
FIG. 1 illustrates a scanning-type semiconductor projection apparatus, which is one embodiment of the present invention. As shown in FIG. 1, illuminating light IL emitted by a light source 1 is acted upon by a mirror 2, a reticle blind 3, a relay lens 4, a mirror 5 and a condenser lens 3 and illuminates a reticle 7 with a uniform illuminance and over a slit-shaped illumination area decided by the reticle blind 3. A reticle stage 8 is supported on a reticle stage surface plate 9, and a reticle interferometer 11 is provided for sensing the position of the reticle stage 8 by projecting a laser beam LB onto a moving mirror 10 on the reticle stage 8 and then receiving the reflected light. The reticle stage 8 is staged to the left and right (along the direction of the y axis) in FIG. 1.
A projection optical system PO is disposed below the reticle stage 8 and projects, in a reduced size, the circuit pattern of reticle 7 onto a wafer W, which is a photosensitive substrate, at a predetermined demagnification. The wafer W is held by a precision stage 12a on the top of a wafer stage 12 moved two-dimensionally in a horizontal plane. The position of the wafer stage 12 can be sensed by using a wafer laser interferometer 14, which irradiates a moving mirror 13 with a laser beam LB and receives the reflected light. The wafer stage 12 is mounted on the wafer stage surface plate 15.
During an exposure operation, the wafer stage 12 is scanned in sync with the reticle stage 8 in a direction opposite that of the reticle stage 8 along the y axis in FIG. 1. Reaction forces produced by driving both stages cause vibration of the structural members of the main body, which include the reticle stage surface plate 9 and wafer stage surface plate 15. This causes vibration also of the projection optical system PO, which is one of the structural members of the main body.
The influence of vibration-induced error that develops in the measurement signals from the reticle interferometer 11 and wafer laser interferometer 14 can be ascertained by feeding back, to the control systems of respective stages, the outputs of acceleration sensors 18R and 18W serving as vibration sensors provided in close proximity to the stages. The feedback arrangement is disclosed in the specification of Japanese Patent Application Laid-Open No. 10-12513 entitled xe2x80x9cScanning-type Projection Exposure Apparatusxe2x80x9d. The vibration sustained by the projection optical system PO is of a mode different from that of vibration of the main body imposed upon the reticle interferometer 11 and wafer laser interferometer 14. The effects of vibration caused by the projection optical system PO upon exposure precision, therefore, cannot be mitigated or eliminated by the technique described in the above-mentioned laid-open specification.
The specification of Japanese Patent Application Laid-Open No. 10-261580 entitled xe2x80x9cProjection Apparatusxe2x80x9d also is publicly known material indicating an arrangement for solving the above-mentioned problem. This publication discloses an apparatus in which vibration of the main body of the exposure apparatus is measured by a vibration sensor, and a vibration-induced error that develops in the measured value from a laser interferometer is corrected for by a main control system using the result of measurement, whereby positional offset of the reticle and wafer is prevented. More specifically, vibration of the main body of the exposure apparatus is sensed by mounting a vibration sensor on the projection optical system, which accommodates a group of projection lenses, and feeding this vibration forward to a position control system that controls the position of a precision stage carrying a reticle.
Though this disclosure does not have any description specifying the position at which the vibration sensor is mounted, the drawings show that the vibration sensor is mounted below a lens barrel. In other words, the intention is to sense vibration at the position of exposure, namely on the side of the wafer. It will be appreciated that an attempt is being made to correct, at the reticle precision stage, the positioning precision whereof declines in proportion to the demagnification of the projection optical system PO, error at the exposure position caused by vibration of the lens barrel.
In FIG. 2 of the above-mentioned disclosure, there is shown a block for correcting the position of the reticle precision stage based upon an output signal from the vibration sensor with which the projection optical system is equipped. FIG. 8 of the present application shows FIG. 2 of the above-mentioned disclosure. The intended operation of the apparatus of FIG. 8 will now be described in detail using nomenclature and reference characters identical with those of the above-mentioned disclosure.
First, the output of an acceleration sensor 50 is converted to a velocity signal by an integrating circuit 70 and the velocity signal is then fed forward to a control system 56 for the reticle precision stage. The feed-forward signal is applied to a physical location (the input side of an integrator 76 in the Figure) representing the velocity of the reticle precision stage. In actuality, the signal can be injected only in front of the driver that drives the reticle precision stage. Accordingly, the depiction of the block diagram in FIG. 8 is in error and a faithful representation of this block diagram is not possible. However, an idea of what is striving to be achieved may be understood from FIG. 8. Specifically, vibration of the projection optical system is sensed by the acceleration sensor 50, absolute displacement of vibration of the projection optical system is calculated by directing the signal indicative of sensed vibration through the integrators 70 and 76, and the absolute displacement is deemed to correspond to a target signal applied to the reticle precision stage in the position control system. That is, a quantity corresponding to displacement caused by vibration of the projection optical system is calculated and the position of the reticle precision stage is corrected using this quantity per se.
Though it is described that the output of the acceleration sensor 50 is xe2x80x9cfed forwardxe2x80x9d to the reticle precision stage, the output is xe2x80x9cfed backxe2x80x9d if a strict interpretation is made based upon the dynamics. The reason for this can be understood by referring to the specification of Japanese Patent Application Laid-Open No. 5-250041 entitled xe2x80x9cPositioning Apparatus with Multiple Acceleration Feedbackxe2x80x9d. Stated simply in strict conformity with the aforesaid Japanese Patent Application Laid-Open No. 10-261580, the projection optical system, which is a structural member, vibrates owing to the vibration of each stage. Conversely, when a structural member vibrates for some reason, this influences the performance of the stages. In other words, the structural members, which include the stages and projection optical system, are linked dynamically. If vibration (a signal indicative of acceleration) of these linked structural members is incorporated in the stage control systems, this can be thought of as feedback that contributes to the stability of the control systems. However, since the position of the reticle precision stage is corrected by this feedback, there is always a proper amount thereof and this amount is very small. As a result, this simply does not demonstrate an operation that will change the values specific to the stage control systems. For these reasons, the introduction of an output from a vibration sensor mounted on the projection optical system to a stage control system shall be referred to as feedback in the specification of this application.
Problems encountered with the arrangement of the apparatus disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 10-261580 (xe2x80x9cExposure Apparatusxe2x80x9d) will now be indicated.
When the generally columnar projection optical system PO rotates about a center of rotation, both translational and rotational displacements are sustained by the group of lenses within the projection optical system. When a certain xe2x80x9ccenter of rotationxe2x80x9d is a xe2x80x9cnodal pointxe2x80x9d on the projection optical system PO, shifts in the exposing light caused by translation and rotation experienced by the group of lenses cancel each other out and do not appear as a shift in the final imaging position on the wafer. In other words, though aberration occurs, correction for displacement of the exposure position is intrinsically unnecessary. Nevertheless, in accordance with the exposure apparatus of Japanese Patent Application Laid-Open No. 10-261580, the position of the reticle precision stage is corrected at all times using the output of the vibration sensor, which is mounted below the projection optical system, even though the projection optical system undergoes rotational vibration at a nodal point. Consequently, an error in the position of the exposing light is brought about by applying a meaningless correction where none is originally required. Japanese Patent Application Laid-Open No. 10-261580, in order to correct for the influence of vibration impressed upon the laser interferometer serving as the means for measuring the position of the stage, aims at measuring the vibration of the projection optical system, which is regarded as vibrating together with the laser interferometer, and applying a correction to the position of the reticle precision stage using a signal output indicative of the measured vibration. A problem which arises is that it is not possible to correct for the influence on exposing light error of vibration of the projection optical system, the aspect whereof is different from that of the laser interferometer.
The problems dealt with in the present invention will now be summarized.
Higher throughput sought when manufacturing semiconductor devices requires the high-speed scanning or high-speed positioning of the reticle stage and wafer stage. Owing to a reaction force produced when driving each stage in such a manufacturing operation, laser interferometers serving as means for measuring the positions of the respective stages rigidly connected to the structure of the main body of the apparatus are caused to vibrate. At the same time, vibration, the aspect of which is different from that of the aforementioned laser interferometers, also occurs in the projection optical system constituting the heart of the semiconductor exposure apparatus.
In the prior art, the effects on stage stabilization of vibration impressed upon the laser interferometers are mitigated or eliminated by control means which corrects for these effects. However, the projection optical system, which comprises a group of projection lens and a lens barrel accommodating these lenses, is generally columnar in shape and is supported mechanically in a vertical attitude. Driving the reticle stage or wafer stage, therefore, causes the projection optical system to undergo primarily wobbling vibration. Since this vibration bends the exposure light beam that passes through the interior of the projection optical system, it leads to poor exposure precision. In other words, an unsolved problem is that means have not yet been fully developed to eliminate the deleterious influence on exposure precision of vibration of the projection optical system the mode of which differs from the mode of vibration of the laser interferometer.
The foregoing is a consideration of the influence of projection optical system vibration upon positional offset of the reticle (original plate) and wafer (substrate to be exposed) in directions lying in a plane (the XY plane) orthogonal to the optic axis of the projection optical system. However, vibration of the projection optical system influences also offset (focal-point deviation) of the wafer in a direction along the optic axis of the projection optical system as well as distortion (aberration) of the image plane.
Thus, in the exposure apparatus according to the prior art, the surface of a wafer, for example, is brought into alignment with the imaging plane of a projection optical system. In order to do this, a focusing deviation or deviation in inclination at the surface of the wafer is measured by a focus inclination sensor and alignment is carried out using a stage that is capable of adjusting for this focusing deviation or deviation in inclination.
However, in such an exposure apparatus according to the prior art, the imaging plane and the wafer surface cannot be aligned with a high precision even if the stage is driven in accordance with a correction quantity calculated from the value measured by the focus inclination sensor. As a result, there is a decline in contrast, resolving power and alignment precision.
The reasons for this are believed to be as follows: Conventionally, the position of the focus inclination sensor relative to the imaging plane of the projection optical system is regarded as being fixed without measuring displacement or distortion of the imaging plane of the projection optical system. The amount by which the stage is to be corrected is calculated from the measured value provided by the focus inclination sensor, and the imaging plane and wafer surface are positioned accordingly. In actuality, however, the imaging plane is displaced or deformed by vibration of the projection optical system. Consequently, even if the stage is driven in accordance with the amount of correction calculated from the value measured by the focus inclination sensor, the displaced and/or distorted imaging plane cannot be brought into accurate alignment with the wafer surface. This leads to deteriorated contrast, resolving power and alignment precision.
Accordingly, an object of the present invention is to provide a projection exposure apparatus in which vibration of the projection optical system as caused by the driving reaction force of each of the stages does not have a deleterious effect upon exposure precision.
Another object of the present invention is to ascertain the state of vibration of the projection optical system in an appropriate fashion and correct for its effects upon exposure error.
Another object of the present invention is to provide a projection exposure apparatus in which the displacement of the imaging plane of the projection optical system is calculated by measuring vibration of the projection optical system and alignment of a substrate surface on a stage is carried out taking the calculated displacement of the imaging plane into account, thereby reducing deviation between the substrate surface and the imaging plane.
Yet another object of the present invention is to improve contrast and resolving power by reducing deviation between the substrate surface and the imaging plane.
A further object of the present invention is to provide a device manufacturing method using a projection exposure apparatus that attains the foregoing objects.
According to the present invention, the foregoing objects are attained by providing a projection exposure apparatus having a projection optical system for projecting a pattern, which has been formed on an original plate, onto a substrate to be exposed, the apparatus including at least two vibration measurement means for measuring vibration of the projection optical system.
According to a more specific first embodiment of the present invention, the projection exposure apparatus comprises an original plate stage on which the original plate is placed and which is movable in a plane orthogonal to the optic axis of the projection optical system; a substrate stage on which the substrate to be exposed is placed and which is movable in a plane orthogonal to the optic axis of the projection optical system; position measurement means for measuring positions of the original plate and substrate to be exposed; and control means for controlling the position of at least one of the original plate and substrate to be exposed based upon a measured value from the position measurement means and measured values from the at least two vibration measurement means.
In this embodiment, vibration sensors or laser interferometers of a number necessary to sense flexural vibration of the projection optical system are provided. In the case of a step-and-repeat exposure apparatus, preferably the control means applies the exposing light after sensing or judging that flexural vibration has fallen within a predetermined tolerance. In the case of a scanning-type projection exposure apparatus, preferably an approach distance ahead of the acceleration/deceleration profile or constant-velocity scan is optimized in order to drive the wafer or reticle or both in such a manner that flexural vibration of the projection optical system will fall within the predetermined tolerance when scanning exposure is performed.
Furthermore, in both the step-and-repeat demagnifying-projection-type semiconductor exposure apparatus and scanning-type projection exposure apparatus, it is preferred that the parameters of a vibration-proof device supporting the projection optical system be optimally selected so as to minimize flexural vibration in the projection optical system.
According to a more specific second embodiment of the present invention, the projection exposure apparatus comprises a substrate stage on which the substrate to be exposed is placed and which is movable along the direction of the optic axis of the projection optical system and in a direction orthogonal to the direction of the optic axis; surface position measurement means for measuring relative position of the surface of the substrate with respect to a predetermined position along the direction of the optic axis of the projection optical system; and control means for moving a prescribed amount and positioning the substrate along any direction orthogonal to the optic axis of the projection optical system using the substrate stage, and moving and positioning the substrate along the direction of the optic axis based upon measured values from the vibration measurement means and a measured value from the surface position measurement means, thereby aligning the surface of the substrate with the predetermined position. In this case, means can be provided for driving the original plate, on which the pattern has been formed, based upon the measured value from the vibration measurement means.
When the original plate stage and/or substrate stage is positioned in the conventional exposure apparatus, despite the fact that vibration of the projection optical system is sensed, only a single vibration sensor is used to sense this vibration, as described in the specification of Japanese Patent Application Laid-Open No. 10-261580, and a problem which arises is that it is not always possible to properly correct for a shift in imaging position in a direction orthogonal to the optic axis caused by vibration of the projection optical system, as mentioned above. By contrast, the present invention provides the exposure apparatus with at least two vibration measurement means so that modes of vibration of the projection optical system can be sensed separately. Accordingly, position and or travel of the original plate stage and/or substrate stage is corrected for in dependence upon the mode of vibration, thereby making it possible to improve precision of alignment of the substrate with respect to the original plate as well as the precision of synchronization of the original plate stage and substrate stage. As a result, the harmful effects of vibration of the projection optical system upon exposure precision can be suppressed.
In the conventional exposure apparatus described above, shift and distortion of the image plane due to vibration of the projection optical system at the time of automatic focusing is not taken into consideration. That is, vibration of the projection optical system is not sensed for the purpose of correcting automatic focusing. By contrast, the present invention is such that a plurality of acceleration sensors are mounted on the projection optical system. Displacement or distortion of the imaging plane of the projection optical system is calculated based upon measured values from these sensors, thereby making it possible to align the surface of the substrate with the imaging plane of the projection optical system in a highly precise fashion. As a result, contrast, resolving power and alignment precision can be improved.
In accordance with another embodiment of the present invention, the foregoing objects are attained by providing a projection exposure apparatus comprising: projection optics for projecting a pattern, which has been formed on an original plate, onto a substrate; a stage on which the substrate is placed and which is movable along the direction of an optic axis of the projection optics and in a direction orthogonal to the direction of the optic axis; surface position measurement means for measuring relative position of the surface of the substrate, which has been placed upon the stage, with respect to a predetermined position decided based upon the position of the projection optics; and vibration measurement means, which has a plurality of vibration sensors, for measuring vibration of the projection optics; and driving means for controlling drive of the stage based upon results of measurement by the surface position measurement means and the vibration measurement means.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.